Hereinafter, background arts of two aspects of the present invention will be described.
In the first aspect of the present invention, an improved structure of an optical multiplexer/demultiplexer filter is described, among optical devices having optical functional elements formed from multilayer dielectrics or the like. The optical multiplexer/demultiplexer comprises an optical branching filter or the like is inserted between the end faces of two collimator lenses, and also input and output ports formed from optical fibers are placed on the other end faces of the collimator lenses.
FIG. 10 shows an example of an optical functional component disclosed in U.S. Pat. No. 6,347,170. An optical functional component 1 in the figure is a WDM (wavelength division multiplexed) type optical multiplexer/demultiplexer, and has a WDM optical multiplexer/demultiplexer filter 3 as the optical functional component. First and second refractive index distribution type lenses 2 and 4 serving as collimating lenses are placed on the two sides of this optical multiplexer/demultiplexer filter 3. The refractive index distribution type lens (also called a GRIN lens) is a substantially cylindrical lens having a radial refractive index, which reduces from the lens optical axis towards the outer periphery.
A first port 5 for input and a second port 6 for output, which are formed from optical fibers, are connected to the end face 2a of the first refractive index distribution type lens 2 on the opposite side from the optical multiplexer/demultiplexer filter 3. Similarly, a third port 7 formed from an optical fiber is connected to the end face of the second collimator lens 4 on the opposite side from the optical multiplexer/demultiplexer filter 3.
The end faces of the respective ports 5, 6 and 7, and the end faces 2a and 4a of the refractive index distribution type lenses 2 and 4, facing the respective ports 5, 6 and 7, are ground and polished so as to be tilted with a predetermined angle (in general within a range from 6 to 8 degrees) to the optical axis, in order to prevent light reflected by the end faces from entering the optical path. As a result, the reflected light is diverted away from the optical axis.
The optical multiplexer/demultiplexer 1 functions as below. At first, when wavelength division multiplexed signal light is input from the first port 5, this light is converged by the first refractive index distribution type lens 2 and input to the optical multiplexer/demultiplexer filter 3. The optical multiplexer/demultiplexer filter 3 passes only light of a certain wavelength, of the wavelength division multiplexed signal light, and reflects the light of other wavelengths. Therefore, the light which can pass through the optical multiplexer/demultiplexer filter 3, after passing therethrough, is output to the third port 7 via the second refractive index distribution type lens 4. Moreover, the light reflected in the optical multiplexer/demultiplexer filter 3, after reflecting therein is output to the second port 6 via the first refractive index distribution type lens 2. In this manner, the incident light incident from the first port 5 can be branched into two outputs which are output from the second and third ports 6 and 7.
With this, in conventional optical function goods as mentioned above, in order to reduce manufacturing cost, refractive index distribution type lenses with the same length and similar specifications are used for the refractive index distribution type lenses 2 and 4. However, since the end faces 2a and 4a thereof are ground diagonally, the optical path length (optical path length AD+DB in FIG. 4) in the lens from the first port 5 to the second port 6 after reflection by the optical multiplexer/demultiplexer filter 3 is not the same as the optical path length (optical path length AD+EC in FIG. 4) in the lens from the first port 5 to the third port 7 after transmission through the optical multiplexer/demultiplexer filter 3. In FIG. 10, points A to E indicate positions where the optical signals are input to or output from the end faces of the refractive index distribution type lenses 2 and 4.
Therefore, since both the output light from point B to the second port 6, and the output light from point C to the third port 7 cannot have the same focal length, the coupling efficiency to the ports 6 and 7 is reduced causing an increase in losses. In particular recently this is a problem with optical multiplexer/demultiplexers for high density wavelength division multiplexed communication systems which are being introduced corresponding to the increase in communication capability.
Heretofore, in order to resolve this problem, the lengths of the refractive index distribution type lenses 2 and 4 are made shorter than the focal distances of the lenses (0.25 pitch), and the end faces of the ports 5, 6, and 7 and the end faces of the respective facing refractive index distribution type lenses 2 and 4 are separated, and the positional relation of these is adjusted (aligned) so that the losses can be reduced as much as possible, and these are then secured using an adhesive.
However, in this method, since alignment with the lens optical axis direction is necessary, alignment of the ports 5, 6 and 7 is difficult and thus takes time. Moreover, since the distance between the ports 5, 6 and 7 and the end faces of the refractive index distribution type lenses 2 and 4 is increased, then in the case of a structure where adhesive is filled between the end faces, there remains the problem that the performance of the optical functional component is susceptible to the influence of volumetric changes due to the mechanical strength and temperature changes of the adhesive, so that the stability of the optical characteristics is deteriorated.
In the second aspect of the present invention, an improved optical multiplexer/demultiplexer components is described, which comprises an optical multiplexer/demultiplexer filter, an optical branching filter or the like is inserted between the end faces of two collimator lenses, and also input and output ports formed from optical fibers are placed on the other end faces of the collimator lenses.
FIG. 11 shows a conventional example schematic structure of an optical multiplexer/demultiplexer (for example, refer to U.S. Pat. No. 6,347,170).
An optical multiplexer/demultiplexer 1 in the figure is provided with an optical multiplexer/demultiplexer element 3 formed from a multilayer dielectric, and first and second collimator lenses 2 and 4 placed on the two sides of this optical multiplexer/demultiplexer element 3. First and second refractive index distribution type lenses 2 and 4 serving as collimator lenses are placed on the two sides of this optical multiplexer/demultiplexer filter 3.
A first port 5 and a second port 6, which are formed from optical fibers, are connected to the end face 2a of the first collimator lens 2 on the opposite side from the optical multiplexer/demultiplexer element 3. Furthermore, a third port 7 formed from an optical fiber is connected to the end face of the second collimator lens 4 on the opposite side from the optical multiplexer/demultiplexer element 3.
In this optical multiplexer/demultiplexer 1, refractive index distribution type lenses with a pitch of approximately 0.25 are used for the first and second collimator lenses 2 and 4. Here, a length of one pitch in a refractive index distribution type lens represents a sinusoidal cycle of a light beam traveling in the refractive index distribution type lens.
Accordingly, the optical path length in the lens from the first port to the second port after reflection by the optical multiplexer/demultiplexer element 3, and the optical path length (not including the optical path length transmitting through space and the optical multiplexer/demultiplexer element 3) in the lens from the first port to the third port after passing through the optical multiplexer/demultiplexer element 3, are both 0.5 pitch. Therefore, light diverged and entered from the first port converges on the end face of the lens, so if the locations of each of the ports 5 through 7 are selected appropriately, it is possible to connect the collimator lenses 2 and 4 and the ports 5 through 7 with high efficiency.
Incidentally, however, in recent years, there has been a strong requirement to expand communication capacity, and hence the wavelength regions used for communication have been expanded, which constitutes problems of the second aspect. Because of wavelength dispersion in the collimator lenses 2 and 4, the differences in focal lengths between wavelengths of optical signals with large differences in wavelength cannot be ignored. This causes outgoing lights from the collimator lenses 2 and 4 to converge at locations deviating from the end faces of the ports 5 through 7, and hence there is a problem of increasing losses.